Taken primarily from Kozloff, 1987, 1996 p. 266 (Copyright 1987, 1996, University of Washington Press. Used in this web page by permission of University of Washington Press)
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| Lower taxonomic level | Main Page | Alphabetic Index | Systematic Index | Glossary |
| 1a | Ligament external; interior of valves porcellanous, not pearly; with a pallial sinus (animal with siphons) | 2 |
| 1b | Ligament internal; interior of valves pearly, especially when fresh; without a pallial sinus | Family Nuculidae |
| 2a | With a chondrophore on the lower part of the hinge plate | 3 |
| 2b | Without a chondrophore | 4 |
| 3a | Shell closing tightly, pallial sinus shallow | Family Nuculanidae |
| 3b | Shell gaping posteriorly; pallial sinus deep | Family Yoldiidae |
| 4a | Series of hinge teeth posterior to the umbo nearly twice as long as the series anterior to the umbo; posterior ends of valves rounded | Family Malletiidae |
| 4b | Series of hinge teeth posterior to the umbo not much longer than the series anterior to the umbo; posterior ends of valves somewhat pointed due to way the the curved ventral margin meets the nearly straight dorsal margin | Family Tindariidae |